High blood pressure, or hypertension, is a major health problem in America an estimated 72 million adults suffer from this condition in the United States. The problem is even greater worldwide.
Feedback from a non-invasive monitor has been shown to improve patient compliance with prescribed treatment regimens. Additionally, a person's blood pressure is not constant throughout the day and is known to exhibit a circadian rhythm. Twenty four hour ambulatory blood pressure monitoring has been shown to be a valuable tool in predicting cardiovascular events.
Leading non-invasive home and hospital blood pressure monitors, however, are not well-suited for non-invasive monitoring because, in that they are based on oscillometry and sphygmomanometery, they rely on inflation of an obtrusive cuff to estimate blood pressure (BP). These cuff based devices require occlusion of the artery being monitored through cuff inflation which increases the external pressure acting on the artery above the systolic blood pressure. During BP measurement, these types of actuated monitors are uncomfortable and interfere with the wearer's daily routine by requiring an irksome measurement period.
A passive, non-invasive blood pressure monitoring alternative exists in devices that measure arterial pulse wave velocity or pulse transit time (PTT). Although this method offers the potential for passive beat-to-beat blood pressure estimation, it has found limited use in home and hospital BP monitoring due to inherent drawbacks in the existing technology.
PTT-based devices are deficient in that they are incapable of autonomous calibration. Typically, a traditional cuff based device is required to calibrate the patient-specific relationship between PTT and BP prior to their use. The relationship between PTT and BP is governed by a number of vascular properties; chief among these is the compliance of the artery. The PTT/BP relationship is discussed in Nichols & O'Rourke, McDonald's Blood Flow in Arteries: Theoretical, Experimental, and Clinical Principles, (4th ed., London, 1998), chap. 3, which is incorporated herein by reference.
The compliance of skeletal arteries may be altered by both central and local cardiovascular control mechanisms, as discussed, for example by Guyton & Hall, Textbook of Medical Physiology, (9th ed., 1996), chap. 18, which is incorporated herein by reference. Use of a fixed, predetermined calibration model can result in significant BP estimation error using PTT measurements if the cardiovascular state of the patient changes.